The present invention relates generally to a pipe union and more particularly to a pipe union which is suitable for both metallic and non-metallic pipes, especially but not exclusively for those of large diameter and subject to high interior pressures.
It is known from the prior art to provide non-metallic pipes, that is pipes of synthetic plastic material and the like, which are connected at their juncture by adhesive connections, by welding or by simple friction couplings using elastic sealing members. The friction couplings, wherein the end portion of one pipe is pushed into that of a pipe with which it is to be connected, is particularly advantageous in terms of the simplicity of establishing such a pipe connection. This is especially helpful where the pipe union must be established in situ, that is on a construction site.
However, the prior-art constructions for friction couplings have certain disadvantages. In order for the elastic sealing element which is accommodated between the connected end portions of the pipes in such couplings, to be fully effective even in the event of high interior pressures in the pipes, it is necessary that the clearance between the two telescoped-together pipe end portions must be of small dimensions. If that is not the case, the elastic sealing element can be pushed out of its proper position by the interior pressure existing in the pipes, and the sealing effectiveness can thereby become lost. Because of the small dimensions permissible for the clearance, the connected pipes can perform only a small relative angular movement, but in many instances the installation conditions are such that angular movements of a more significant extent are required. Moreover, in these prior-art constructions any relative angular movement of the pipes results in stress-transmission between the pipes which can lead to destruction of the pipe material, because the free edge of the inner pipe end portion is wedged against a part of the inner circumferential surface of the outer pipe end portion, so that high pressures are transmitted over a small surface area, amounting, in effect, to transmission by point contact. For this latter reason, in particular, it has heretofore been necessary to install pipes which are connected in this manner, with exceedingly great care. If, as is frequently the case, the pipes are to be located at the bottom of a pipe ditch, then it was necessary to place the pipes onto the bottom of the ditch and to connect them only after they had been put in place. This, on the other hand, necessitated that the ditch had to be much wider than would otherwise have been the case, in order to permit access of the operators to the pipes. Needles to say, the wider the ditch must be made, the more work is involved in digging it and the more expensive the installation will become.
A further disadvantage of these prior-art telescoped pipe connections has been that they do not provide for retention of the pipes against axial separation. This could be overcome, for instance in cases where the pipes are installed in a ditch, by providing a requisite number of abutments which engage the pipes and prevent their axial displacement. However, a large number of such abutments would have to be provided, and the larger the pipe diameter and the interior pressure to which the pipes are subjected, the stronger would be the forces tending to separate the pipes axially and the larger would have to be the abutments.
An attempt has been made in the prior art to avoid this last problem by using a telescoped-together pipe connection which is provided with passages in the connected end portions, which passages so cooperate with one another that when, after the telescoping-together of the pipe end portions, a length of steel rope or wire rope is inserted into these passages, the two pipes will be locked against relative axial displacement. This, also, has certain disadvantages. On the one hand, if the pipes are of synthetic plastic material which does not corrode, the use of the wire rope which does readily corrode introduces a factor which can significantly reduce the maintenance-free lifetime of a pipe installation. Moreover, if the pipes are of large diameter and/or subjected to high internal pressures, the use of this type of connection is in any case of questionable validity, both in terms of the economics involved and in terms of the structural strength. In addition, if pipes connected in this manner undergo relative angular movement, which in actual practices can hardly ever be excluded, the forces which are transmitted between the pipes via the wire rope act upon very small surface areas of the respective pipes, so that there is again the danger that the pipe material might become damaged or even destroyed.
Another type of pipe union known from the prior art provides for a snap-coupling of the pipe end portions to be connected, wherein one pipe end portion is forced into the other pipe end portion of the adjacent pipe, which is possible because the synthetic plastic material of which these pipes are made is capable of yielding sufficiently to permit such introduction. An adhesive or a sealing material is then injected into spaces which are provided between the outer circumferential surface of the inner pipe end portion and the inner circumferential surface of the outer pipe end portion.
It is evident that this type of pipe union also is possessed of certain disadvantages. In the first place, it can be practiced only with pipes of synthetic plastic material, and with pipes of relatively thin walled construction and of small dimensions, because this type of connection relies on the resilient deformation of the pipe material. Furthermore, angular displacement of the connected pipes with reference to one another is neither possible nor permissible because such displacements would break the seal between the two connected pipe end portions. Moreover, such a coupling can be successfully used only if the pipe end portions to be connected are dimensionally very accurate, which in many instances is a very difficult or even impossible requirement. It is clear that tolerance variations under such circumstances will readily lead to leakage at the pipe union. Aside from all this, it will be appreciated that the use of adhesives for injection in the manner outlined above, is not readily practical for in situ installations, because even relatively minor ambient influences, for instance dust, moisture, strong sunlight or the like, can so influence the quality of the adhesive connection as to jeopardize the integrity and sealing effectiveness of the pipe union. Despite these disadvantages, the use of adhesive connections for joining small-diameter pipes has become relatively widespread, but adhesive connections for large-diameter pipes have never been used in practice.
As has been pointed out earlier, it is also known in the prior art to weld end portions of respective pipes together. Of course, this would in almost all instances have to be done in situ, that is at the building site, and this requires the presence of skilled personnel and of relatively complicated and expensive equipment. Aside from that, a connection by welding is limited to only certain types of pipe materials and is relatively slow and therefore economically not attractive.
In addition to the various disadvantages outlined above with respect to the prior art, all of the prior-art pipe unions have the further drawback that if they develop a leak, the location of the lead along a line of pipes can be established only with great difficulty, and by time-consuming procedures. It is not only necessary that each pipe union be completely accessible at all points of its circumference, but also the specific manner in which the tests for tightness must be carried is relatively primative and involves the use of bubble-blowing material or the like.